1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases that contain nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons.
2. Description of the Related Art
For the purification treatment of exhaust gases from automobiles and other similar sources, catalysts (composition: Pt, Rh/Al.sub.2 O.sub.3 system) known as three-way catalysts which utilize hydrocarbons and CO in the automotive emissions are usually used. The three-way catalysts are normally prepared by applying a .gamma.-alumina slurry to a refractory carrier of cordierite or the like, baking, and then causing the carrier to support a metal, e.g., Pt, Rh, or Pd. They cannot remove NOx, hydrocarbons, and CO, however, unless the combustion occurs within a very narrow range close to the theoretical air-fuel ratio (e.g., A/F=14.6).
With growing concern over the global environments in recent years, there has been strong demand for fuel-efficient cars. Accordingly, lean-burn engines for combustion above the theoretical air-fuel ratio have arrested attention as a key technology. It is well-known that the fuel efficiency is improved by burning in the engine a fuel mixture of an increased air-fuel ratio (A/F).
However, this is accompanied by an increase in the oxygen concentration in the exhaust to such a level that while an ordinary three-way catalyst can remove hydrocarbons and CO, it does not remove NOx.
To purify the exhaust gas in the "lean-burn" region where the oxygen concentration is high, it has already been proposed to use catalysts consisting of a transition metal, such as Cu, supported by a zeolite of the general formula, xM.sub.2 /.sub.n O.Al.sub.2 O.sub.3.ySiO.sub.2, wherein M is an n-valent metal (see, e.g., Japanese Patent Provisional Publications Nos. 125250/1985 (60-125250) and 130735/1989 (1-130735)). Another catalyst has also been introduced to meet the end which comprises a crystalline silicate to which Cu is added, the silicate itself having a chemical composition, (1.+-.0.4)R.sub.2 O.[aM.sub.2 O.sub.3.bAl.sub.2 O.sub.3 ].ySiO.sub.2, in which R is an alkali metal ion and/or hydrogen ion, M is the ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, and antimony, a+b=1.0, a.ltoreq.0, b&gt;0, and y&gt;12 (Japanese Patent Provisional Publication No. 303194/1989 (1-303194)).
While these recently introduced catalysts exhibit satisfactory initial activities, they have the disadvantage of limited durability.
Exhaust gases from ordinary lean-burn engines are at high temperatures over a broad range of 300.degree. to 700.degree. C., and conventional catalysts used for such engines have been found to deteriorate due to sintering of the supported active metal. Also, the present inventors have found that the catalytic deterioration of such conventional catalysts is accelerated by demetallization, a phenomenon in which aluminum and iron in the crystal lattices, which form ion exchange sites in the catalyst carrier zeolite and crystalline silicate, are removed from the crystal lattices under the influence of the high temperature exhaust gas atmosphere, especially in the presence of steam.